Slow to respond and attachments

Want to start of by saying i am new to zimbra so if this is a dumb question i apologize.

we are having speed issues with our zimbra server. Attachments are taking an extremely long time to download and often times our users who are using outlook get an error message saying that An operation on the server timed out. The server may be down, overloaded, or there may be too much net traffic.

Unfortunatly i am not sure. I just took the job here about 4 weeks ago and it has been happening since i have been here. the mail server is running on a vmware fusion instance on a mac machine. the mac machine does have noobproof installed on it. all imap, pop3 ports are all set to allow.